Up to the present, capacitors, particularly electric double layer capacitors having a capacity intermediate between that of a battery and that of a capacitor, as a backup power supply for IC's and memories and a supplement or alternative to secondary batteries, have been used widely as a low-power direct-current power supply. In recent years, however, there have been new portable electronic devices coming into being one after another, such as VTR with a built-in camera, cellular phones and laptop computers. Amid this trend, there has been a growing demand that those electric double layer capacitors used as a backup power supply and a supplement or alternative to secondary batteries should have higher energy density.
These electric double layer capacitors, unlike batteries, do not convert a chemical change into electric energy. They utilize a large capacity of the electric double layer that occur between the electrodes and the electrolytic solution to put in and out an electric charge in the same manner as the charge and discharge of batteries. Such electric double layer capacitors are normally so constructed that they use a non-corrosive electrolytic solution that does not corrode the capacitor container or the collector made of aluminum, copper or the like and two electrodes formed from a material having a large surface area such as activated carbon and a binder such as fluororesins are so arranged that they face each other via a porous separator made of polyethylene or polypropylene.
As the electrolytic solution for such electric double layer capacitors, aqueous-solution-based electrolytic solution and organic-solvent-based electrolytic solution (non-aqueous electrolytic solution) are used. However, aqueous-solution-based electrolytic solution showing low potential window of electrochemical stability (approx. 1.2 V) are difficult to provide high-energy-density electric double layer capacitors.
By contrast, compared with aqueous-solution-based electrolytic solution, organic-solvent-based electrolytic solution (non-aqueous electrolytic solution) show high potential window of electrochemical stability and therefore make the formation of high-energy-density capacitors possible. Because of this, the electric double layer capacitors using non-aqueous electrolytic solution are rapidly beginning to come into use as a backup power supply for electronic equipment for non-military use.
As such non-aqueous electrolytic solution, a mixture of a non-aqueous solvent such as cyclic carbonic ester exhibiting a high dielectric constant and an electrolyte such as tetraethylammonium tetra fluoroborate, for example, is used.
However, such an electrolytic solution as mentioned above has had the problem of the inner electrical resistance of the A capacitor going up due to its low electric conductivity, making it difficult to obtain high-output capacitors. Furthermore, when future capacitors have high energy density, the aforementioned electrolytic solution might be insufficient in potential window of electrochemical stability. Because of this, it has been hoped that a non-aqueous electrolyte having better charging/discharging cycle properties will come into existence.